Final answer:
The rough endoplasmic reticulum (rough ER) is the site where proteins intended for export from the cell are synthesized.
Step-by-step explanation:
The structure that is the site of the synthesis of proteins that may be exported from the cell is the rough endoplasmic reticulum (rough ER). The rough ER is distinguished by the presence of ribosomes on its surface, which synthesize proteins for export outside the cell or for insertion into the cell membrane. These proteins often carry a signal peptide that directs the growing protein to dock at the rough ER, where it is translocated across the ER membrane into the lumen. Once synthesized, proteins are packaged into transport vesicles which later fuse with the Golgi apparatus for further modification before being sent to their final destinations, including export out of the cell.